Switch failure detection device, battery pack including the same, and method of detecting failure of electronic switch

ABSTRACT

A switch failure detection device includes a switch, a rectifier, a switch voltage detection circuit, and a controller. The switch is connected in a path in which a charging current to and a discharging current flow. The rectifier passes a discharging current by bypassing the switch when the switch is turned off. The detection circuit detects a voltage between an input and an output of the switch. The controller is configured to: determine whether the electric storage device is in a discharging state; send an off-command signal to the switch if the electric storage device is in the discharging state; receive the voltage; determine an input-output voltage of the switch based on the voltage; determine whether the input-output voltage is lower than a first reference voltage; and determine the switch has a turn-off problem if the input-output voltage is lower than the first reference voltage.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No.2012-45676 filed on Mar. 1, 2012.

TECHNICAL FIELD

The technology described in this specification relates to detection offailure of a switch configured to shut off charging or dischargingcurrents.

BACKGROUND

An electric storage device such as a lithium-ion battery may beovercharged or over-discharged due to variations in capacity betweencells or a failure of a peripheral device such as a charger and a load.Therefore, a known battery pack may include an overcharge/over-dischargeprotection function. Such a battery pack may include an electronicswitch such as an FET and a monitoring unit. The switch is connected ina current path in which a charging current or a discharging currentflows. The monitoring unit is configured to measure a terminal voltageof the electric storage device. If the terminal voltage reaches apredetermined level during charging of the electric storage device, theswitch is turned off to shut off the charging current so that theelectric storage device is protected against overcharge.

A known battery pack including a protection function may have thefollowing configuration. If a voltage between an input and an output ofan electronic switch, that is, a voltage drop during charging is large,an abnormal condition in which an on resistance is excessively high dueto a failure of the switch is determined. If the abnormal condition isdetermined, the switch is turned off to disable a battery.

In such a battery pack, the switch may not be turned off if a shortcircuit occurs between the input and the output of the switch or forsome reasons. If the switch cannot be turned off, the protectionfunction may not be properly performed. As a result, the electricstorage device may be overcharged or over-discharged. Even if anabnormal condition of the switch is detected, the battery cannot bedisabled because the switch cannot be turned off.

A turn-off problem of the switch may be detected in advance by turningthe switch from on to off. However, power supply to a load may bestopped if switch failure detection is performed as such on adischarging current shut-off switch during discharging. If the switchfailure detection is performed on a charging current shut-off switchduring charging, power supply to the electric storage device may bestopped.

SUMMARY

A switch failure detection device may be installed in an electric systemincluding an electric storage device. The switch failure detectiondevice includes at least one electronic switch, at least one rectifier,a switch voltage detection circuit, and a controller.

The electronic switch is to be connected in a path in which a chargingcurrent to the electric storage device and a discharging current fromthe electric storage device flow. The electronic switch is switchedbetween on and off. The rectifier is configured to pass a dischargingcurrent by bypassing the electronic switch when the electronic switch isturned off. The switch voltage detection circuit is configured to detecta voltage between an input and an output of the electronic switch.

The controller is configured to: determine whether the electric storagedevice is in a discharging state; send an off-command signal to theelectronic switch to turn off the electronic switch if the electricstorage device is in the discharging state; receive the voltage detectedby the switch voltage detection circuit while the off-command signal issent to the electronic switch; determine an input-output voltage of theelectronic switch based on the voltage; determine whether theinput-output voltage is lower than a reference voltage; and determinethe electronic switch has a turn-off problem in which the electronicswitch does not turn off according to the off-command signal if theinput-output voltage is lower than the reference voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a battery pack according to anembodiment.

FIG. 2 is a flowchart of battery protection process.

FIG. 3 is a flowchart of a switch failure detection process.

FIG. 4 is a circuit diagram of the battery pack with a charging currentshutoff FET turned off.

DETAILED DESCRIPTION

An embodiment will be described with reference to FIGS. 1 to 4. Abattery pack 1 includes a secondary battery 2 and a battery protectioncircuit 3. The battery pack 1 may be installed in an electric vehicle ora hybrid vehicle and supply power to various devices in the vehicle. Thesecondary battery 2 is an example of an electric storage device. Acapacitor may be used instead of the secondary battery 2. The batteryprotection circuit 3 is an example of a switch failure detection device.The battery pack 1 may include a connector for electrically connectingthe secondary battery 2 to an external device.

Electric Configuration of Battery Pack

The secondary battery 2 is a lithium-ion battery and an assembledbattery including four cells 2A connected in series. Alternatively, thesecondary battery 2 may include only one cell 2A. Further alternatively,the secondary battery may include two series-connected cells 2A, threeseries-connected cells 2A, or five or more series-connected cells 2A.

The battery protection circuit 3 includes first to fourth connectionterminals T1 to T4, a charging current shutoff FET 31, a dischargingcurrent shutoff FET 32, and a battery monitoring unit 33. The secondarybattery 2 is connected between the first and the second connectionterminals T1 and T2. External devices including a charger 5 and a load 6are selectively connected between the third and the fourth connectionterminals T3 and T4 via a selector 7.

The charging current shutoff FET 31 and the discharging current shutoffFET 32 are N-channel MOSFETs including parasitic diodes D1 and D2,respectively. The charging current shutoff FET 31 and the dischargingcurrent shutoff FET 32 are examples of switches and rectifiers. Thedrain of the charging current shutoff FET 31 and that of the dischargingcurrent shutoff FET 32 are commonly connected. Namely, the chargingcurrent shutoff FET 31 and the discharging current shutoff FET 32 areconnected back-to-back. The source and the gate of the charging currentshutoff FET 31 are connected to the connection terminal T3 and thebattery monitoring unit 33, respectively. The source and the gate of thedischarging current shutoff FET 32 are connected to the connectionterminal T1 and the battery monitoring unit 33, respectively.

The battery monitoring unit 33 includes a controller 34, a first voltagedetection circuit 35, and a second voltage detection circuit 36. Thecontroller 34 includes a central processing unit (CPU) 34A and a memory34B. The memory 34B stores various programs that are provided forcontrolling operations of the battery monitoring unit 33. The CPU 34Areads out the programs from the memory 34B and controls the componentsof the battery monitoring unit 33 according to the programs. The memory34B includes RAM and ROM. The medium on which the programs are stored isnot limited to RAM. A non-volatile memory such as CD-ROM, hard diskdrive, and a flash memory can be used.

The first voltage detection circuit 35 is an example of switch voltagedetection circuit. The first voltage detection circuit 35 detects afirst voltage V1 between the first and the second connection terminalsT1 and T2. The first voltage V1 is an example of a terminal voltage ofthe electric storage device. The first voltage detection circuit 35sends a detection signal corresponding to the first voltage V1 to thecontroller 34. The first voltage V1 is proportional to a terminalvoltage of the secondary battery 2. The second voltage detection circuit36 detects a second voltage V2 between the third and the fourthconnection terminals T3 and T4. The second voltage detection circuit 36sends a detection signal to the controller 34. The second voltage V2 isproportional to an output voltage of the charger 5 or a voltage acrossthe load 6.

Controls on Battery Monitoring Unit

When the battery protection circuit 3 is turned on, the CPU 34A startssending on-command signals to the charging current shutoff FET 31 andthe discharging current shutoff FET 32. The on-command signals are forturning on, or closing, the charging current shutoff FET 31 and thedischarging current shutoff FET 32. The charging current shutoff FET 31and the discharging current shutoff FET 32 remain turned on whilereceiving the on-command signals.

When the selector 7 is thrown to a charger position and connectionbetween the third connection terminal T3 and the charger 5 isestablished, the power is supplied from the charger 5 to the secondarybattery 2. As a result, the secondary battery 2 is charged. When theselector 7 is thrown to a load position and connection between the thirdconnection terminal T3 and the load 6 is established, the secondarybattery 2 starts discharging, and the power is supplied from thesecondary battery 2 to the load 6 (see FIG. 1). The CPU 34A reads outthe programs from the memory 34B and performs a battery protectionprocess illustrated in FIG. 2 and a switch failure detection processillustrated in FIG. 3.

Battery Protection Process

In the battery protection process, the CPU 34A continuously orperiodically detects the first voltage V1 based on the detection signalfrom the first voltage detection circuit 35 (S1). The CPU 34A determineswhether the first voltage V1 is higher than an overcharge threshold(S2). A preferable level of the overcharge threshold is slightly lowerthan the first voltage V1 when the secondary battery 2 is in anovercharged state and slightly higher than the first voltage V1 when thesecondary battery 2 is in an over-discharged state. The overchargethreshold may be defined in advance based on an experiment in which thesecondary battery 2 is set in the overcharged state and the firstvoltage V is measured.

If the first voltage V1 is higher than the overcharge threshold (YES inS1), the CPU 34A performs an overcharge control step to send anoff-command signal to the charging current shutoff FET 31 (S3). The CPU34A performs the overcharge control step because the secondary batterymay enter the overcharged state. The charging current shutoff FET 31 isturned off, that is, opened, and the charging current from the charger 5is shut off by the parasitic diode D1. This terminates the charge of thesecondary battery 2. Therefore, the secondary battery 2 is less likelyto enter the overcharged state. The CPU 34A returns to step S1 when theovercharge control step is complete.

If the first voltage V1 is lower than an overcharge threshold (NO inS2), the CPU 34A determines whether the first voltage V1 is lower thanthe over-discharge threshold (S4). The over-discharge threshold may bedefined in advance based on an experiment in which the secondary battery2 is set in the over-discharged state and the first voltage V ismeasured.

If the first voltage V1 is lower than the over-discharge threshold (YESin S4), the CPU 34A performs an over-discharge control step to send anoff-command signal to the discharging current shutoff FET 32 (S5). TheCPU 34A performs the over-discharge control step because the secondarybattery may enter the over-discharged state. The discharging currentshutoff FET 32 is turned off and the discharging current from thesecondary battery 2 is shut off by the parasitic diode D2. Thisterminates the discharge of the secondary battery 2. Therefore, thesecondary battery 2 is less likely to enter the over-discharged state.The CPU 34A returns to step 91 when the over-discharge control step iscomplete.

If the first voltage V1 is higher than the over-discharge threshold (NOin S4), the CPU 34A returns to step S1. In this case, the CPU 34Acontinues sending the on-command signals to the charging current shutoffFET 31 and the discharging current shutoff FET 32.

Switch Failure Detection Process

The CPU 34A executes the switch failure detection process illustrated inFIG. 3 if a specific condition is satisfied while the on-command signalsare sent to the charging current shutoff FET 31 and the dischargingcurrent shutoff FET 32. The specific condition may be power-up of thevehicle or predetermined elapsed time since the previous switch failuredetection process.

The CPU 34A determines whether the secondary battery 2 is in thedischarging state (S11). The CPU 34A may determine the state of thesecondary battery 2 based on an instruction signal from an enginecontrol unit (ECU, not illustrated) or the charger 5. The dischargingstate includes a state in which the secondary battery 2 outputs a smallcurrent such as a dark current after the load 6 is disconnected. Thecurrent protection circuit 3 may include a current sensor to detect thedischarging current. With this configuration, the current protectioncircuit 3 may determine the discharging state based on the current, morespecifically, a direction of current, detected by the current sensor.

Switch Failure Detection Process for Charging Current Shutoff FET 31

If the secondary battery 2 is in the discharging state (YES in S11), theCPU 34A detects a first on voltage VON1 between the first connectionterminal T1 and the third connection terminal T3 (S12). The on voltageVON1 is an example of a terminal voltage. Specifically, the on voltageVON1 is a voltage drop between the FETs 31 and 32 while the on-commandsignals are sent to the FETs 31 and 32. In this embodiment, a voltagedifference between the first voltage V1 and the second voltage V2 iscalculated based on the detection signals from the first voltagedetection circuit 35 and the second voltage detection circuit 36, anddefined as the on voltage VON1.

The CPU 34 determines whether the on voltage VON1 is lower than a firstthreshold TH1 (S13). The first threshold TH1 is an example of a secondreference value. A preferable level of the first threshold TH1 isslightly higher than the on voltage VON1 that is detected when thevoltage detection circuits 35 and 36 and the FETs 31 and 32 are notdefective and able to perform proper operations.

If the on voltage VON1 is lower than the first threshold TH1 (YES inS13), the CPU 34A sends the off-command signal to the charging currentshutoff FET 31 to turn off the charging current shutoff FET 31 (S14).The CPU 34A turns off the charging current shutoff FET 31 because thevoltage detection circuits 35 and 36 and the FETs 31 and 32 are inconditions to perform proper operations if the on voltage VON1 is lowerthan the first threshold TH1. As illustrated in FIG. 4, a forwarddirection of the parasitic diode D1 corresponds to a direction in whicha discharging current I flows. Therefore, the discharging current Icontinues flowing into the load 6 via the parasitic diode D1 after thecharging current shutoff FET 31 is turned off.

The CPU 34A detects a first off voltage VOFF1 between the firstconnection terminal T1 and the second connection terminal T3 (S15). Theoff voltage VOFF1 is an example of a voltage between an input and anoutput of an electronic switch. Specifically, the off voltage VOFF1 is avoltage drop between the FETs 31 and 32 during sending of theoff-command signal to the charging current shutoff FET 31 and theon-command signal to the discharging current shutoff FET 32. In thisembodiment, a voltage difference between the first voltage V1 and thesecond voltage V2 is calculated based on the detection signals from thefirst voltage detection circuit 35 and the second voltage detectioncircuit 36, and defined as the off voltage VOFF1. The first voltagedetection circuit 35 and the second voltage detection circuit 36 areexamples of a switch voltage detection circuit.

The CPU 34A sends the on-command signal to the charging current shutoffFET 31 to turn the charging current shutoff FET 31 back on (S16). Apreferable period in which the off-command signal is sent to thecharging current shutoff FET 31 is several milliseconds or as short aspossible. By turning off the charging current shutoff FET 31,instability in power supply to the load 6 can be reduced.

The CPU 34A determines whether a first on-off voltage difference ΔV1 islower than a second threshold TH2 (S17). The on-off voltage differenceΔV1 is an example of an input-output voltage determined by a controllerbased on a voltage detected by the switch voltage detection circuit.Specifically, the on-off voltage difference ΔV1 is a voltage differencebetween the on voltage VON1 and the off voltage VOFF1. The secondthreshold TH2 is an example of a first reference voltage. A preferablelevel of the second threshold TH2 slightly lower than the voltagedifference between the on voltage VON1 and the off voltage VOFF1detected in advance when the FETs 31 and 32 are not defective and ableto perform proper operations.

If the on-off voltage difference ΔV1 is lower than the second thresholdTH2 (YES in S17), the CPU 34A executes first error processing (S18). TheCPU 34A executes the first error processing because the charging currentshutoff FET 31 has a turn-off problem in which the charging currentshutoff FET 31 does not turn off according to the off-command signal dueto a short circuit between input and output or for some reason. In thefirst error processing, the CPU 34 issues a notification about theturn-off problem of the charging current shutoff FET 31 to the externaldevices including the ECU. When the first error processing is complete,the CPU 34 terminates the switch failure detection process. If theon-off voltage difference ΔV1 is higher than the second threshold TH2(NO in S17), the CPU 34A terminates the switch failure detection processwithout executing the first error processing.

If the on voltage VON1 is higher than the first threshold TH1 (NO inS13), the CPU 34A executes second error processing (S19). The CPU 34Aexecutes the second error processing because at least one of thecircuits 35 and 36 is defective or at least one of the FETs 31 and 32 isdefective and thus the switch failure detection process may not beproperly executable. In the second error processing, the CPU 34 issues anotification about the failure of at least one of the circuits 35 and 36or at least one of the FETs 31 and 32 to the ECU. When the second errorprocessing is complete, the CPU 34 terminates the switch failuredetection process.

Switch Failure Detection Process for Discharging Current Shutoff FET 32

If the secondary battery 2 is not in the discharging state, that is, inthe charging state (NO in S11), the CPU 34A detects an on voltage VON2between the first connection terminal T1 and the third connectionterminal T3 (S20). The on voltage VON2 is an example of a terminalvoltage. Specifically, the on voltage VON2 is a voltage drop between theFETs 31 and 32 while the on-command signals are sent to the FETs 31 and32. In this embodiment, a voltage difference between the first voltageV1 and the second voltage V2 is calculated based on the detectionsignals from the first voltage detection circuit 35 and the secondvoltage detection circuit 36, and defined as the on voltage VON2.

The CPU 34A determines whether the on voltage VON2 is lower than a thirdthreshold TH3 (S21) to determine whether the switch failure detectionprocess is properly executable on the discharging current shutoff FET32. The third threshold TH3 is an example of a second reference voltage.A preferable level of the third threshold TH3 is slightly higher thanthe on voltage VON2 that is detected when the voltage detection circuits35 and 36 and the FETs 31 and 32 are not defective and able to performproper operations.

If the on voltage VON2 is lower than the third threshold TH3 (YES inS21), the CPU 34A sends the off-command signal to the dischargingcurrent shutoff FET 32 to turn off the discharging current shutoff FET32 (S22). The CPU 34A turns off the discharging current shutoff FET 32because the switch failure detection process is properly executable ifthe on voltage VON2 is lower than the third threshold TH3. A forwarddirection of the parasitic diode D2 corresponds to a direction in whicha charging current flows. Therefore, the charging current continuesflowing into the second battery 2 via the parasitic diode D2 after thedischarging current shutoff FET 32 is turned off.

The CPU 34A detects an off voltage VOFF2 between the first connectionterminal T1 and the third connection terminal T3 (S23). The off voltageVOFF2 is an example of a voltage between an input and an output of anelectronic switch. Specifically, the off voltage VOFF2 is a voltage dropbetween the FETs 31 and 32 during sending of the on-command signal tothe charging current shutoff FET 31 and the off-command signal to thedischarging current shutoff FET 32. In this embodiment, a voltagedifference between the first voltage V2 and the second voltage V2 iscalculated based on the detection signals from the first voltagedetection circuit 35 and the second voltage detection circuit 36, anddefined as the off voltage VOFF2.

The CPU 34A sends the on-command signal to the discharging currentshutoff FET 32 to turn the discharging current shutoff FET 32 back on(S24). A preferable period in which the off-command signal is sent tothe discharging current shutoff FET 32 is several milliseconds or asshort as possible. By turning off the discharging current shutoff FET32, instability in charging of the second battery 2 can be reduced.

The CPU 34A determines whether a second on-off voltage difference ΔV2 islower than a fourth threshold TH4 (S25). The on-off voltage differenceΔV2 is an example of an input-output voltage determined by a controllerbased on a voltage detected by the switch voltage detection circuit.Specifically, the on-off voltage difference ΔV2 is a voltage differencebetween the on voltage VON2 and the off voltage VOFF2. The fourththreshold TH4 is an example of a first reference value. A preferablelevel of the fourth threshold TH4 slightly lower than the voltagedifference between the on voltage VON2 and the off voltage VOFF2detected in advance when the FETs 31 and 32 are not defective and ableto perform proper operations.

If the on-off voltage difference ΔV2 is lower than the fourth thresholdTH4 (YES in S25), the CPU 34A executes third error processing (S26). TheCPU 34A executes the third error processing because the dischargingcurrent shutoff FET 32 may have a turn-off problem in which thedischarging current shutoff FET 32 does not turn off according to theoff-command signal if the on-off voltage difference ΔV2 is lower thanthe fourth threshold TH4. In the third error processing, the CPU 34issues a notification about the turn-off problem of the dischargingcurrent shutoff FET 32 to the external devices including the ECU. Whenthe third error processing is complete, the CPU 34 terminates the switchfailure detection process. If the on-off voltage difference ΔV2 ishigher than the fourth threshold TH4 (NO in S25), the CPU 34A terminatesthe switch failure detection process without executing the third errorprocessing.

If the on voltage VON2 is higher than the third threshold TH3 (NO inS21), the CPU 34A executes the second error processing (S19), andterminates the switch failure detection process. The CPU 34A executesthe second error processing because the switch failure detection processcannot be properly performed.

In this embodiment, the turn-off problem is determined if the detectedon-off voltage difference ΔV1 is lower than the second threshold TH2under the condition that the off-command signal is sent to the chargingcurrent shutoff FET 31 while the second battery 2 is in the dischargingstate. The discharging current I from the secondary battery 2 continuesflowing into the load 6 via the parasitic diode D1 after the chargingcurrent shutoff FET 31 is turned off. Therefore, the turn-off problem ofthe charging current shutoff FET 31 can be detected while the powersupply to the load 6 is maintained.

The turn-off problem is also determined if the detected on-off voltagedifference ΔV2 is lower than the fourth threshold TH4 under thecondition that the off-command signal is sent to the discharging currentshutoff FET 32 while the second battery 2 is in the charging state. Thecharging current from the charger 5 continues flowing into the secondarybattery 2 via the parasitic diode D2 after the discharging currentshutoff FET 32 is turned off. Therefore, the turn-off problem of thedischarging current shutoff FET 32 can be detected while the charging ofthe secondary battery 2 by the charger 5 is maintained. The switchfailure detection process is performed in both cases in which thesecondary battery 2 is in the charging state and in the dischargingstate by controlling the FETs 31 and 32. In comparison to aconfiguration in which the FETs 31 and 32 are connected in differentpaths, the number of conductive lines or monitoring devices can bereduced. Namely, the configuration of this embodiment is simpler.

If the on-off voltage differences ΔV1 and ΔV2 are lower than therespective thresholds TH2 and TH4, the turn-off problem is determined.In comparison to a configuration in which the turn-off problem isdetermined if the off voltage VOFF1 or VOFF2 is lower than a threshold,accuracy in detection of the turn-off problem is less likely to bereduced even the on-resistance of the FET 31 or 32 varies when the FET31 or 32 becomes defective.

If the FET 31 or 32 becomes defective and the on-resistance thereofincreases or the voltage detection circuit 35 or 36 becomes defective,the turn-off problem may not be properly detected. In this embodiment,the switch failure detection process is executed if the on voltage VON1or VON2 is lower than the corresponding first threshold TH1 or TH3.Therefore, the switch failure detection process is less likely to beexecuted in the condition that the turn-off problem is not properlydetected.

Other Embodiments

The scope of the present invention is not limited to the aboveembodiment. The following embodiments are also included in the scope ofthe technologies described herein.

The controller 34 may include a plurality of CPUs or a hardware circuitsuch as an application specific integrated circuit (ASIC).Alternatively, the controller 34 may include both hardware circuit andCPU. At least two of the overcharge control step, the over-dischargecontrol step, the determination of the state, the sending of theoff-command signal, the determination of proper execution of the switchfailure detection process, and the determination of the turn-off problemmay be performed by different CPUs or hardware circuits. The sequence inwhich the above operations are performed may be altered.

The switches may be bipolar transistors, relays, or any types ofswitches that do not include parasitic diodes. The rectifiers may bediodes or diode-connected transistors. An output of one of thediode-connected transistors is connected to an input of another of thediode-connected transistors. However, the embodiment described earliercan perform the switch failure detection process using existingcomponents without additional components.

The controller 34 may be configured to determine whether the switchfailure detection process is properly executable based on a voltage ofone of the cells or voltages of the cells in the secondary battery 2.

The controller may be configured to execute the switch failure detectionprocess for only one of the charging current shutoff FET 31 and thedischarging current shutoff FET 32.

The input-output voltage may be the off voltage VOFF1 or the off voltageVOFF2.

What is claimed is:
 1. A switch failure detection device to be installedin an electric system including an electric storage device, the switchfailure detection device comprising: at least one electronic switch tobe connected in a path in which a charging current to the electricstorage device and a discharging current from the electric storagedevice flow and switched between on and off; at least one rectifierconfigured to pass a discharging current by bypassing the electronicswitch when the electronic switch is turned off; a switch voltagedetection circuit configured to detect a voltage between an input and anoutput of the at least one electronic switch; and a controllerconfigured to: determine whether the electric storage device is in adischarging state; send an off-command signal to the at least oneelectronic switch to turn off the at least one electronic switch if theelectric storage device is in the discharging state; receive the voltagedetected by the switch voltage detection circuit while the off-commandsignal is sent to the at least one electronic switch; determine aninput-output voltage of the at least one electronic switch based on thevoltage; determine whether the input-output voltage is lower than areference voltage; and determine the at least one electronic switch hasa turn-off problem in which the at least one electronic switch does notturn off according to the off-command signal if the input-output voltageis lower than the reference voltage.
 2. The switch failure detectiondevice according to claim 1, wherein the voltage detected by the switchvoltage detection circuit includes an on voltage and an off voltage, theon voltage being a voltage between the input and the output of the atleast one electronic switch detected while an on-command signal to turnon the at least one electronic switch is sent to the at least oneelectronic switch, the off voltage being a voltage between the input andthe output of the at least one electronic switch detected while theoff-command signal is sent to the at least one electronic switch, andthe controller is further configured to: send an on-command signal tothe at least one electronic switch to turn on the at least oneelectronic switch; and receive the on voltage detected by the switchvoltage detection circuit while the on-command signal is sent to the atleast one electronic switch.
 3. The switch failure detection deviceaccording to claim 2, wherein the controller is further configured todetermine the input-output voltage of the at least one electronic switchbased on a difference between the on voltage and the off voltage.
 4. Theswitch failure detection device according to claim 2, wherein thereference voltage is a first reference voltage, and the controller isfurther configured to: determine whether the on voltage is lower than asecond reference voltage; and cancel the determining whether the voltageis lower than the first reference voltage if the on voltage is higherthan the second reference voltage.
 5. The switch failure detectiondevice according to claim 1, wherein the at least one electronic switchincludes a first electronic switch and a second electronic switch, theat least one rectifier includes a first rectifier and a secondrectifier, the first rectifier being connected parallel to the firstelectronic switch such that a forward direction thereof corresponds to adirection in which the discharging current flows, the second rectifierbeing connected parallel to the second electronic switch such that aforward direction thereof corresponds to a direction in which thecharging current flows, the switch voltage detection circuit is furtherconfigured to detect a first voltage between an input and an output ofthe first electronic switch and a second voltage between an input and anoutput of the second electronic switch, the reference voltage is a firstreference voltage, and the controller is further configured to: send theoff-command signal to the first electronic switch and an on-commandsignal to the second electronic switch to turn on the second electronicswitch if the electric storage device is in the discharging state; sendan on-command signal to the first electronic switch to turn on the firstelectronic switch and the off-command signal to the second electronicswitch if the electric storage device is in the charging state; receivethe first voltage detected by the switch voltage detection circuit whilethe off-command signal is sent to the first electronic switch; determinea first input-output voltage based on the first voltage; determinewhether the first input-output voltage is lower than the first referencevoltage; determine the first electronic switch has a turn-off problem inwhich the first electronic switch is not turned off according to theoff-command signal sent to the first electronic switch if the firstinput-output voltage is lower than the first reference voltage; receivethe second voltage detected by the switch voltage detection circuitwhile the off-command signal is sent to the second electronic switch;determine a second input-output voltage based on the second voltage;determine whether the second input-output voltage is lower than thefirst reference voltage; and determine the second electronic switch hasa turn-off problem in which the second electronic switch is not turnedoff according to the off-command signal sent to the second electronicswitch if the second input-output voltage is lower than the firstreference voltage.
 6. The switch failure detection device according toclaim 1, wherein the at least one electronic switch includes a firstelectronic switch and a second electronic switch, the at least onerectifier includes a first rectifier and a second rectifier, the firstrectifier being parasitic to the first electronic switch such that aforward direction thereof corresponds to a direction in which thedischarging current flows, the second rectifier being parasitic to thesecond electronic switch such that a forward direction thereofcorresponds to a direction in which the charging current flows, theswitch voltage detection circuit is further configured to detect a firstvoltage between an input and an output of the first electronic switchand a second voltage between an input and an output of the secondelectronic switch, the reference voltage is the first reference voltage,and the controller is further configured to: send the off-command signalto the first electronic switch and an on-command signal to the secondelectronic switch to turn on the second electronic switch if theelectric storage device is in the discharging state; send an on-commandsignal to the first electronic switch to turn on the first electronicswitch and the off-command signal to the second electronic switch if theelectric storage device is in the charging state; receive the firstvoltage detected by the switch voltage detection circuit while theoff-command signal is sent to the first electronic switch; determine afirst input-output voltage based on the first voltage; determine whetherthe first input-output voltage is lower than the first referencevoltage; determine the first electronic switch has a turn-off problem inwhich the first electronic switch is not turned off according to theoff-command signal sent to the first electronic switch if the firstinput-output voltage is lower than the first reference voltage; receivethe second voltage detected by the switch voltage detection circuitwhile the off-command signal is sent to the second electronic switch;determine a second input-output voltage based on the second voltage;determine whether the second input-output voltage is lower than thefirst reference voltage; and determine the second electronic switch hasa turn-off problem in which the second electronic switch is not turnedoff according to the off-command signal sent to the second electronicswitch if the second input-output voltage is lower than the firstreference voltage.
 7. A switch failure detection device to be installedin an electric system including an electric storage device, the switchfailure detection device comprising: at least one electronic switch tobe connected in a path in which a charging current and a dischargingcurrent flow and switched between on and off; at least one rectifierconfigured to pass a charging current by bypassing the electronic switchwhen the electronic switch is turned off; a switch voltage detectioncircuit configured to detect a voltage between an input and an output ofthe at least one electronic switch; and a controller configured to:determine whether the electric storage device is in a charging state;send an off-command signal to the at least one electronic switch to turnoff the at least one electronic switch if the electric storage device isin the charging state; receive the voltage detected by the switchvoltage detection circuit while the off-command signal is sent to the atleast one electronic switch; determine an input-output voltage of the atleast one electronic switch based on the voltage; determine whether theinput-output voltage is lower than a reference voltage; and determinethe at least one electronic switch has a turn-off problem in which theat least one electronic switch does not turn off according to theoff-command signal if the input-output voltage is lower than thereference voltage.
 8. The switch failure detection device according toclaim 7, wherein the voltage detected by the switch voltage detectioncircuit includes an on voltage and an off voltage, the on voltage beinga voltage between the input and the output of the at least oneelectronic switch detected while an on-command signal to turn on the atleast one electronic switch is sent to the at least one electronicswitch, the off voltage being a voltage between the input and the outputof the at least one electronic switch detected while the off-commandsignal is sent to the at least one the electronic switch, and thecontroller is further configured to: send an on-command signal to the atleast one electronic switch to turn on the at least one electronicswitch; and receive the on voltage detected by the switch voltagedetection circuit while the on-command signal is sent to the at leastone electronic switch.
 9. The switch failure detection device accordingto claim 8, wherein the controller is further configured to determinethe input-output voltage of the at least one electronic switch based ona difference between the on voltage and the off voltage.
 10. The switchfailure detection device according to claim 8, wherein the referencevoltage is a first reference voltage, and the controller is furtherconfigured to: determine whether the on voltage is lower than a secondreference voltage; and cancel the determining whether the voltage islower than the first reference voltage if the on voltage is higher thanthe second reference voltage.
 11. The switch failure detection deviceaccording to claim 7, wherein the at least one electronic switchincludes a first electronic switch and a second electronic switch, theat least one rectifier includes a first rectifier and a secondrectifier, the first rectifier being connected parallel to the firstelectronic switch such that a forward direction thereof corresponds to adirection in which the discharging current flows, the second rectifierbeing connected parallel to the second electronic switch such that aforward direction thereof corresponds to a direction in which thecharging current flows, the switch voltage detection circuit is furtherconfigured to detect a first voltage between an input and an output ofthe first electronic switch and a second voltage between an input and anoutput of the second electronic switch, the reference voltage is a firstreference voltage, and the controller is further configured to: send theoff-command signal to the first electronic switch and an on-commandsignal to the second electronic switch to turn on the second electronicswitch if the electric storage device is in the discharging state; sendan on-command signal to the first electronic switch to turn on the firstelectronic switch and the off-command signal to the second electronicswitch if the electric storage device is in the charging state; receivethe first voltage detected by the switch voltage detection circuit whilethe off-command signal is sent to the first electronic switch; determinea first input-output voltage based on the first voltage; determinewhether the first input-output voltage is lower than the first referencevoltage; determine the first electronic switch has a turn-off problem inwhich the first electronic switch is not turned off according to theoff-command signal sent to the first electronic switch if the firstinput-output voltage is lower than the first reference voltage; receivethe second voltage detected by the switch voltage detection circuitwhile the off-command signal is sent to the second electronic switch;determine a second input-output voltage based on the second voltage;determine whether the second input-output voltage is lower than thefirst reference voltage; and determine the second electronic switch hasa turn-off problem in which the second electronic switch is not turnedoff according to the off-command signal sent to the second electronicswitch if the second input-output voltage is lower than the firstreference voltage.
 12. The switch failure detection device according toclaim 7, wherein the at least one electronic switch includes a firstelectronic switch and a second electronic switch, the at least onerectifier includes a first rectifier and a second rectifier, the firstrectifier being parasitic to the first electronic switch such that aforward direction thereof corresponds to a direction in which thedischarging current flows, the second rectifier being parasitic to thesecond electronic switch such that a forward direction thereofcorresponds to a direction in which the charging current flows, theswitch voltage detection circuit is further configured to detect a firstvoltage between an input and an output of the first electronic switchand a second voltage between an input and an output of the secondelectronic switch, the reference voltage is a first reference voltage,and the controller is further configured to: send the off-command signalto the first electronic switch and an on-command signal to the secondelectronic switch to turn on the second electronic switch if theelectric storage device is in the discharging state; send an on-commandsignal to the first electronic switch to turn on the first electronicswitch and the off-command signal to the second electronic switch if theelectric storage device is in the charging state; receive the firstvoltage detected by the switch voltage detection circuit while theoff-command signal is sent to the first electronic switch; determine afirst input-output voltage based on the first voltage; determine whetherthe first input-output voltage is lower than the first referencevoltage; determine the first electronic switch has a turn-off problem inwhich the first electronic switch is not turned off according to theoff-command signal sent to the first electronic switch if the firstinput-output voltage is lower than the first reference voltage; receivethe second voltage detected by the switch voltage detection circuitwhile the off-command signal is sent to the second electronic switch;determine a second input-output voltage based on the second voltage;determine whether the second input-output voltage is lower than thefirst reference voltage; and determine the second electronic switch hasa turn-off problem in which the second electronic switch is not turnedoff according to the off-command signal sent to the second electronicswitch if the second input-output voltage is lower than the firstreference voltage.
 13. A battery pack comprising: an electric storagedevice; and the switch failure detection device according to claim 1.14. A battery pack comprising: an electric storage device; and theswitch failure detection device according to claim
 7. 15. A method ofdetecting a failure of an electronic switch in an electric systemincluding an electric storage device, the electronic switch, and arectifier, the electronic switch being connected in a path in which acharging current to the electric storage device and a dischargingcurrent from the electric storage device flow and being switched betweenon and off, the rectifier being configured to pass a discharging currentby bypassing the electronic switch when the electronic switch is turnedoff, the method comprising: determining whether the electric storagedevice is in a discharging state; sending an off-command signal to theelectronic switch to turn off the electronic switch if the electricstorage device is in the discharging state; detecting a voltage betweenan input and an output of the electronic switch while the off-commandsignal is sent to the electronic switch; determining an input-outputvoltage of the electronic switch based on the detected voltage;determining whether the input-output voltage is lower than a referencevoltage; and determining the electronic switch has a problem in whichthe switch dose not turn off according to the off-command signal if theinput-output voltage is lower than the reference voltage.
 16. A methodof detecting a failure of an electronic switch in an electric systemincluding an electric storage device, the electronic switch, and arectifier, the electronic switch being connected in a path in which acharging current to the electric storage device and a dischargingcurrent from the electric storage device flow and being switched betweenon and off, the rectifier being configured to pass a charging current bybypassing the electronic switch when the electronic switch is turnedoff, the method comprising: determining whether the electric storagedevice is in a charging state; sending an off-command signal to theelectronic switch to turn off the electronic switch if the electricstorage device is in the charging state; detecting a voltage between aninput and an output of the electronic switch while the off-commandsignal is sent to the electronic switch; determining an input-outputvoltage of the electronic switch based on the detected voltage;determining whether the input-output voltage is lower than a referencevoltage; and determining the electronic switch has a problem in whichthe switch dose not turn off according to the off-command signal if theinput-output voltage is lower than the reference voltage.